1. Field of the Invention
The present invention relates generally to electronic ballasts, and more specifically to cost effective ways of controlling ground-.fault currents in an electronic ballast.
2. Description of the Prior Art
Incandescent lamps are relatively inexpensive compared to fluorescent lamp systems, but fluorescent lamps are more cost effective to operate because they can produce the same amount of light with much less electrical energy. A significant part of the initial cost of a fluorescent lamp system is the ballast. Fluorescent tubes express a negative resistance characteristic that must be controlled in order to maintain consistent light output and long life. Ballasts are also used to start the gaseous discharge tubes and to generate the required voltages, which are substantially higher than ordinary 110 VAC.
The use of integrated circuit technology has provided for lower cost of ballasts and therefore the cost of a fluorescent lamp system. Ballasts are produced in very high quantities and a savings of one or two dollars per ballast can account for millions of dollars of savings and can tip the balance between a choice of incandescent or fluorescent lights. Generally, increasing the number of functions integrated onto a chip lowers the cost of a ballast.
Electronic ballasts often require circuitry that limits potential shock currents from disconnected lamp pins. Industry standards such as Underwriters Laboratories (UL) specification #935 outline shock current limits, in terms of maximum magnitude and duration, which are tested through a resistor connected to earth ground. The resistor simulates some of the important electrical properties of the human body as a short circuit under worst case conditions.
A prior art method to limit such shock currents is to provide an isolation transformer in either the power input or output circuitry of a ballast. (For example, see, U.S. Pat. No. 4,277,726, issued to R.V. Burke.) Such a solution is costly and adds a great deal of bulk to a system. Even high-quality isolation transformers have an insertion loss and generate heat, such that they waste some electrical power, thereby defeating the principal benefit of a fluorescent lamp system.
On Mar. 26, 1985, O.K. Nilssen was issued U.S. Pat. No. 4,507,698, for an inverter type ballast with ground-fault protection. In FIG. 1 of Nilssen '698, a ground-fault transformer GFT has two primary and one secondary winding. Transformer GFT is described as being wound on a Magnetics OJ-41003-TC-00 ferrite torroidal core with fifteen turns of #30 wire for each primary winding and twenty-five turns of #34 for the secondary winding. A ground-fault is expressed as a net 33K Hz push-push current that induces a voltage in the secondary across a resistor R1 and energy storing capacitor EC. The turns ratio of transformer GFT and the value of resistor R1 are chosen to give the circuit a proper degree of sensitivity and freedom from false triggering.
On Jul. 3, 1990, O.K. Nilssen was issued U.S. Pat. No. 4,939,427, for a ground-fault protected series resonant ballast. .This ballast has an EMI filter positioned between the inverter and power line which also acts to produce a control signal in case a ground-fault current occurs. In FIG. 2 of Nilssen '427, a winding AW on an inductor SI with a saturating core has to develop enough voltage across a diode Rc during a ground-fault condition to turn-on a transistor FET which, in turn, limits any ground-fault current. A storage capacitor SC maintains a ground-fault cutoff. A load resistor Rxy is connected across winding AW to desensitize the protection and raise the ground-fault current detection threshold.
The prior art ground-fault protection must be desensitized to noise and imbalanced currents that result during normal operation. Voltage transients can trigger the ground-fault protection and no mechanism is included to verify if a ground-fault condition exists.
Thus there is a need for a more sensitive method of ground-fault current detection and better immunity to stray transients in a ground-fault protected electronic ballast.